Test nested block comments and lexer line tracking

## Why

Wave supports nested `/* ... */` comments and updates line state while skipping comments. This behavior is currently untested in the lexer crate and affects every later diagnostic.

## Scope

- Test line comments, block comments, and nested block comments.
- Verify the line number of the token immediately after single- and multi-line comments.
- Verify whitespace and CRLF handling.
- Add an unterminated block comment case and assert its structured E1002 diagnostic.
- Do not change the comment syntax.

## Likely files

- `front/lexer/src/trivia.rs`
- A new lexer integration test

## Completion criteria

- [ ] Nested comments of at least two levels are covered.
- [ ] Line tracking after comments and CRLF input is asserted.
- [ ] Unterminated comments retain E1002 and useful help text.
- [ ] `cargo test -p lexer --locked` passes.

Start with front/lexer/src/trivia.rs and inspect the existing lexer integration-test layout. Run cargo test -p lexer --locked before adding coverage for line comments, nested block comments, whitespace, CRLF input, and the token line after each comment. Done means the tests assert nested comments, line tracking, and the structured E1002 diagnostic with useful help text.

Written by the indexing model from the issue text.